The present invention relates generally to digital radio communication. More particularly, the present invention relates to a method and apparatus for implementing pseudo-noise (PN) masks in a spread spectrum radio communication system such as a code division multiple access (CDMA) cellular telephone system
Spread spectrum radio systems have been implemented in cellular telephone and other radio systems. In a spread spectrum communication system such as a direct sequence CDMA (DS-CDMA) system, all base stations and mobile stations in all cells of the system use the same radio frequency or band of frequencies for communication. One known DS-CDMA system is defined in Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS95, "Mobile Station-Base Station Compatibility Standard For Dual-Mode Wideband Spread Spectrum Cellular System" (IS-95A).
In addition to traffic channels carrying voice and other data, each radio in the system broadcasts a pilot channel. The pilot channel transmitted by a base station is commonly received by all mobile stations within range. The base pilot is shared between all mobiles in the cell and is used to obtain fast acquisition of new multipaths and for estimation of channel phase and multipath strength). The base station uses the pilot transmitted by a mobile for multipath searches, tracking, coherent demodulation, and to measure the quality of the link for power-control purposes.
The pilot channel transmitted by each radio in the system uses the same repeating PN sequence but with a different phase offset. Transmitters are uniquely identified by using a unique starting phase or starting time for the PN sequences. For example, in IS-95A, the sequences are of length 2.sup.15 chips and are produced at a chip rate of 1.2288 Mega-chips per second (Mcps) and thus repeat every 262/3 milleseconds. The minimum time separations are 64 chips in length, allowing a total of 512 different PN code phase assignments in an IS-95A system.
For next generation systems, higher chip rates are planned. Chip rates of n*1.2288 Mcps are planned, where n is 3 or greater. It is proposed to use a truncated m-sequence generated by the following polynomial: EQU P(x)=2.sup.20 +2.sup.9 +2.sup.5 +2.sup.3 +1
The sequence length is N=2.sup.20 -1=1048575.
Using a chip rate of 3.times.1.2288 Mcps as an example, the number of chips for a period of the pilot PN sequence is 3.times.2.sup.15. There are two sequences required, the in-phase (I) sequence and the quadrature phase (Q) sequence. The in-phase sequence is generated by the following linear recursion:
i(n)=i(n-20).sym.i(n-17).sym.i(n-15).sym.i(n-11), for n=1 to 3.times.2.sup.15, with the initial state equal to PA1 [i(-19),i(-10), . . . ,i(0)]=[00,000,000,000,000,000,001] PA1 q(n)=q(n-20).sym.q(n-17).sym.q(n-15).sym.q(n-11), for n=1 to 3.times.2.sup.15, with the initial state equal to PA1 [q(-19),q(-10), . . . ,q(0)]=[00,000,000,000,000,000,001]
The quadrature phase sequence is generated by
The respective PN sequences are generated by a PN generator. To produce a specific code phase for a particular radio at a particular time, a mask is applied to the output of the PN generator.
There is a problem with the truncated m-sequence proposed for next generation CDMA systems. The problem arises because the sequence is not cyclic or circular. In IS-95A, the length 2.sup.15 sequence repeats every 262/3 ms. A mask used to shift the sequence returned automatically to the beginning of the sequence. If the boundary of the sequence was reached, the sequence and the shifting mask automatically reverted to the origin of the sequence, referred to as sequence rollover or a rollover boundary. In the proposed next-generation sequence, since the sequence is not cyclic, some control of the mask must be provided to ensure that the mask returns to the beginning of the sequence at the rollover boundary.
Accordingly, there is a need for an improved method and apparatus for implementing PN masks for use with a truncated m-sequence.